Process for producing polyvinyl alcohol filaments of improved properties



Patented Sept. 16, 1952 irnfoons's ronraonuome POLYVINYL ALCOHOL FILAMENTS F IMPROVED.

ROP RT E Y 7 William R. Hatehard and Julian .W. Hill, .Wil-

mington, DeL, assignors to E. I. du Pont de Nemours &' Company, Wilmington, DeL, a corporation of Delaware g; Application April 22, 1950,

Serial No. 157,632

eclaims, (c1. 18-54) W This invention relates to the production of synthetic fibers. More particularly, it relates to the production of novel fibers from polyvinyl alcohol.

It is known that polyvinyl alcohol can be: spun into fibers which, when oriented by drawing, possess high tensile strength. In addition, these fibers have the advantages of insolubility in or.- ganic solvents, e. g. dry cleaning solvents, and. of a high softening point which permits ironing when dry. However, they have the. great disadvantage of water-sensitivity Ordinary polyvinyl alcohol yarn dissolves rapidly in .water, particularly at elevated temperature. Thishas counteracted theadvantages mentioned abOvetO the extent that polyvinyl alcoholafibers have found no practical utilityin the textile industry. Methods have been proposed whereby the .water sensitivity of the fiber can be eliminated or considerably reduced by :chemical treatment with an insolubilizing agent. are expensive and sometimes .introduce undesirable physical characteristics in;. the textile made from the treated fibersh There-was. therefore a need for polyvinyl alcohol fibers possessing re sistance' to water and, in particular, resistance to shrinkage therefrom, evenat the boiling point, and free from the objectionable aspectsofadded insolubilizing agents.

This invention has as an object the production of a highly oriented, high tenacity polyvinyl al.-

cohol fiber which, without chemical aftertreatment, is resistant to boiling water. Afurther However, suchtreatments object is a process for preparing such fibers.

Other objects will appear hereinafter.

. ;For the sake oi?v accuracy, a water-resistant polyvinyl .alcohol fiber 'is here defined as one which shrinks lessthan 10% of its length when immersed under substantially no tension in water at 100 C. during a six-minute test. No previous sample of polyvinylalcohol has heretofore been known to give av fiber capable of withstandin such a test without chemical aftertreatment. Fibers from ordinary polyvinyl alcohol are cornpletely dissolved by. water, even below'the boiling point,.within six minutes. I y I M The resistance to water of polyvinyl alcohol fibers may also be tested in another way, involving the temperature at which they lose-10% of their dry tenacity, or tensile strength, upon immersion in water for ten minutes. This .temperature is called the 10% tenacity, ortensile, loss temperature. For thefibers of .this invention it is in generalabove 95 C. In the great majority of cases, the fibers-of this invention shrink less than 6% when immersed in water"at-'- thirty seconds.

100 C. for six minutes, and their 10% tenacity loss temperature is atleast 100 C. and usually at least 102 C. This therefore represents a preferred embodiment of the invention.

'I'he'above-mentioned objects are accomplished by the process of the present invention wherein vinyl formate is polymerized by bringing the samein contact with a polymerizationcatalyst in a substantially anhydrous liquidsystem, preferably containing an organic diluent miscible with the monomer, the polymerization is interrupted (a) before the concentration of the polymer in the polymerization mixture exceeds thereof,.and (h) before the conversion of .monomer to polymer exceeds the vinyl .formate polymer so formed is hydrolyzed substantially completely to polyvinyl alcohol, i. e., to polyvinyl alcohol of saponification number not exceeding 10, the polyvinyl alcohol so formed is spun from aqueous solution to form a filament, the filament is oriented by stretching to a length at. least three times as great as that of the spun filament, washed and dried and the dried, oriented filament further oriented by stretching. at atemperature between 200and 240 0.170 a final'length at least two and one-half times the previous oriented length. vThethus oriented filament may, if desired, advantageously be relaxed by heating the same at atemperature between 210 and 255 C. but not more than 10 C. below the temperature of the second orienting, under tension controlled to prevent shrinkage of more than 35%, and until the filaments shrink not more than 10% when immersed for six minutes in water at C. but in any case fornot over While shrinkage is not necessary, it is preferred that the conditionsof tension be .controlledto permit some shrinkage to take place. This step in practice is effected by runningthe filament from a feed roll .to take-up roll and heating thev filament between the rolls, with the take-up roll rotating at a speed ranging from thesame speed as that of the feed roll to a speed 35% slower than that of the feed roll. At the first-mentioned speed ratio essentially no shrinkage will take place, and at the second-mentioned speed ratio shrinkage will take place up to 35% of its length. In'all instances'the filament will vbe under some tension because of its inherent tendency to shrink (relax) when heated. The

tension is greater when the rolls are at the same peripheral speed anddecreases as the peripheral speed of the take-up roll decreases relative to that of the feed roll.

The principles underlying the extraordinary ventional procedures.

parts of alpha,

3 resistance to Water of the fibers obtained by the present process are not clearly understood. There are, however, some factors of apparent special significance that may be noted in connection with the following more particular reference to the foregoing steps. A polyvinyl ester of particular quality, which is obtained by the first step and which is a polyvinyl formate of high linearity, i. e., characterized by a minimum of chain branching, is apparently necessary. This is indicated by the fact that when a suitable polyvinyl formate of this kind, 1. e., a polyvinyl formate having the characteristics just mentioned, is hydrolyzed, the ratio of its degree of vinyl formate polymerized without the special precautions noted above, approaching unity in the most favorable cases, and the ratio of the molecular weight approaches the theoretical values of 1.64. Accordingly, the ratio of the intrinsic viscosity of the polyvinyl alcohol to that of the polyvinyl formate is much higher than usual for materials prepared according to con- To obtain a polyvinyl formats that will yield a polyvinyl alcohol which by the subsequent steps can be converted into the water resistant fibers, it is necessary, as pointed out in connection with the second step, that the polymerization of the vinyl formate be interrupted at the stated .ipolymer conversion and polymer concentration in the mixture. In general, the polyvinyl formate thus obtained will have a relative viscosity in chloroform at 0., Within the range of 1.03 to 1.18 at a concentration of 0.1%.

In the second step, the 'polyvinvl formate is at least 99% hydrolyzed, i. e., hydrolyzed to the extent that the saponification number of the resulting water-soluble polyvinyl alcohol does not exceed 10. An important factor is the molecular weight of the polyvinyl alcohol, which apparently should not be too high. To remove the uncertainties attending molecular weight determinations of high polymers, it is preferable'to adopt as a criterion the relative viscosity in a-suitable solvent. It has beenfou-nd that the polyvinylale cohols suitable for the production of water-resistant fibers have a" relative viscosity in the range of 1.1 to 1.3 in 85% phenol at 25 C. and 0.1% concentration. --However, polyvinyl alcohol of this viscosity range in not in itself sufficient but must be coupled with the previously mentioned requirement of interrupting the polymerization of the vinyl ester at the specified point because polyvinyl alcohol within the vis cosity range just mentioned can be prepared from polyvinyl forma'te made at veryhigh-polymer conversion, but this polyvinyl alcohol does not yield water-resistant fibers when subjected to the'remaining steps ofthe present process.

The invention is illustrated in further detail by the following examples in'which the parts are by weight.

Example 1 In a three-necked flask fitted with "a watercooled condensen'stirrer, and nitrogen inlet tube were placed 550 parts of. vinyl formate, 11. 1.3838, and 82.5 parts of isopropyl alcohol. After the apparatus had been flushed with nitrogen and the mixture had been heated to refiux,'1,252 alpha-azobisisobutyronitrile (0.1 mole per cent based on the vinyl formate) was added. The polymerization mixture was maintained at the reflux temperature, 47-49" 0.,

4 for two and one-half hours, at the end of which time the polymerization was stopped by the addition of .a small quantity of m-dinitrobenzene. ,After the further addition of several hundred parts of dioxane, the excess vinyl formate was removed by distillation. The dioxane solution of polymer was then poured into a large volume of benzene to precipitate the polymer which was then collected and squeeezed dry. The polymer was further purified by solution in dioxane, reprecipitation in benzene and milling at 120 C. The hard, resinous solid thus obtained Weighed 175 parts (31.8%, conversion based on the vinyl formats). A 0.1% solution of the polymer in chloroform had a relative viscosity of 1.037.

A solution of 150 parts of the above polymer in a mixture of 3,000 parts of purified dioxane and 900 parts of methyl alcohol was heated to reflux in a three-necked flask fitted with a stirrer, condenser and dropping funnel.

To the refluxing solution was added dropwisea mixture of parts by'volume of.1.74 N sodium methoxide solution in methanol and'70 parts of dioxane. Polyvinyl alcohol began to precipitate almost immediately. Themixture was heated under reflux for two hours, warmed below reflux'ior 18 hours, andthen allowed to "cool. The polyvinyl alcohol was collected on a filter, slurriedin methanol, filtered, again slurried in methanoL'filtered and dried at C.'for. 18 hours, when it weighed 96 parts. A 0.1% solution of this polyvinyl alcohol in phenol had a relative viscosity of 1.143. The product had a saponification number of 0.35. The ratio of. the intrinsic viscosity of the polyvinyl alcohol to that of the polyvinyl formate was 3.75. (Intrinsic viscosity has its usual meaning, as defined, for example, in Advances in Colloid Science? vol. II, pageJ209,-or in U. S. Patent 2,130,948, i.-'e., itis the logarithm of the relative viscosity divided .by the concentration in grams per 100 "00.) This high ratio indicates high linearity in the-polymer.

l Molecular weights of the polyvinylformate and its derived polyvinyl alcohol were determined by the osmotic method and found to be 92,100 and 42,750, respectively. These values give a D2 P. (degree of polymerization) ratio of 1.28.

An aqueous solution was prepared containing 16.8% of the above polyvinyl alcohol and 3.1% of pyridine. This solution was spun through a spinneret having 60 holes of 0.004" diameter into a coagulating bath at 26 C. consisting-of a 45% aqueous solution of monosodium phosphate containing 002% of asurface-active and antisticking agent containing 20% of octadecyltrimethylammonium bromide as the activeing'redient. After 60inches of bathtravel involving-pass'ag'e around two yarn driven-rollers,--the yarn was wound arounda *Godet wheel running at a peripheral speed'of 377"per minute. From the Godet wheel the yarn was passed throughair to a windup bobbin'running at "1385" per minute, thereby imparting stretch at a draw ratio of 3.67/1. This amount' oistretch is desirable to prevent sticking during-the subsequent washing with water and drying.

The yarn was washed thoroughly onthe bobbin with cold water to remove the salt deposited on it from the coagulating bath, air-dried'at room temperature iand twisted 1.2 turns per inch. Just prior to'heat stretching, the yarn was'dried by running it over a hotsurface at loo- C. The yarn was then oriented by stretching itin fairthrough a 4" tube 32" 1ong heated to 220 C.

by means'of a jacketcontainingthe vapors of a boiling liquid heat-transfer agent, which in this case. was an eutectic mixture of diphenyl and diphenyl ether. The input speed was '50 feet per minute and the draw ratio was 3.8/1.

The yarn so obtained was resistant to boiling waterand it had a tenacity loss temperature of 103 C. It was further improved by subjecting it to a heat-relaxing step in which the yarn was run through the same tube maintained at a temperature of 235 C. at an input speed of 75 feet per minute, shrinking to the extent of 11% being permitted and controlled by run-'- ning-the take-up roll at'a 11% slower peripheral speed than the feed roll. It then'shrank only 4% when immersed in water at 100 C., and it hada 10% tenacity loss temperature of slightly over. 110 C. Its dry tenacity was 8.5 g./d. .at 10% elongation andits wet tenacity was 7.6 g./d. at 11% elongation.

.For purpose of comparison, a medium viscosity, highly saponified polyvinyl alcohol from standard commercial production was spun from a 14% aqueous-solution as described in Example I,.given a preliminary cold stretch at a draw ratio of 3.4/1, washed,,dried, hot drawn 2.8/1 at 220. C. and relaxed at 220 C. The resulting yarn, which had a dry tenacity of 7.2 g./d. with 17% elongation, dissolved completely in water at 91C. within six minutes. It. should be observed, in connection with the above experiments, that yarns made from polyvinyl alcohol of high linearity, as described herein, can be drawn to appreciably higher draw ratios thanyarns made from the usual types of polyvinyl alcohol.

Example II in 1500 parts of dioxane and 400 parts of methanol was added slowly to a refluxing mixture of 1500 parts of dioxane, 400 parts of methanol and 70 parts by volume of 1.74 N sodium methoxide solution in methanol, and the mixture was stirred under reflux for 16 hours. After filtering, washing with methanol and drying at 70 C., the resulting polyvinyl alcohol 88 parts) had a relative viscosity in 85% phenol at 0.1% concentration'of 1.119, and a saponification number of 4.2. The ratio of the intrinsic viscosity of the polyvinyl alcohol to that of the polyvinyl formatewas 3.44.

- .'An aqueous solution was prepared containing 16.5% of the above polyvinyl alcohol, 3% of pyridine and 0.14% of a long-chain dispersible phosphate ester. This solution, which had a viscosity of about 40 poises was wet spun as described in Example I except that the Godet wheel and wind-up speeds were 313 and 1164 inches per minute, respectively. The yarn was washed and dried as in .Example .I. The yarn was drawn at235 C. to a ratio of. 4.4/1 by the method described in Example In The yarn so obtained shrank only 3% in boiling water, andit had a 10% tenacity loss temperature of 102 C. These properties wereimproved by subjecting the yarn to a heat-relaxing step whereby it was permitted Example III 7 A mixture of 550 parts of vinyl formate, 275 parts of methanol and 1.74 parts of alpha. alpha azobis(alpha,gamma dimethylvaleronitrile) was heated under reflux at 45 50" C.'for 1 /2 hours. The polymerization was interrupted by addition of a little m-dihitrobenzene and the methanol and excess monomer were removed by distillation. Theresidual polymer was purified by twice dissolving it in dioxane and reprecipitating in benzene, followed by milling at 140 C. There was obtained 152 parts (27.5% conversion) of polyvinyl formate'having a relative viscosity in chloroform at 0.1% concentration of 1.038 and a number average. molecularweight of 144,500 by the osmotic pressure method.. I

The above polyvinyl formate (140 parts) was converted to polyvinyl alcohol by alkaline methanolysis in a refluxing mixture of 3000-parts of dioxane and 800 parts of methanol to which was added partsof a 1.7 N solution ofsodium methoxide in. methanol. vAfter 18 hours re fluxing the mixture was neutralized with acetic acid and filtered. The polyvinyl alcohol was slurried several times with methanol, filtered and dried at 65 C. It had a relative viscosity in 0.1% solutionin phenol of 1.170 wide number average molecular weight of 42,350. The ratio of the intrinsic viscosity of the polyvinyl alcohol to' that of the polyvinyl formate was 4.25. The D. P. ratio was 2.09.

An aqueous solution was prepared containing 14.3% of the above polyvinyl alcohol, 0.14% of a long chain, dispersible phosphate ester and about 3.4% of pyridine. This solution was spun as in Example I except that the Godet wheel and wind up speedswere 266 and '968-in'che's per minute, respectively, providing a draw ratio of 36:1. The yarn was washed and dried as in Example I, then heat-stretched at 225 C. as in Example I, with a draw ratio of 4:1. The yarn after this treatment was insoluble in boiling water and its dry tenacity was' 10.4 g./d.= at 4.1% elongation. It'Was then'relaxed 10% at 225 C. by the procedure of ExampleI. After the heat-relaxing step the yarn shrank only 4% in boiling water. Its 10% tenacity losstemperature was 96 C. Its dry tenacity was 6.4 g./d. at 7.5% elongation. 1 The polymerization catalysts used in thefirst step for. polymerizing the monomeric vinyl formate are preferably the free radical-producing catalysts or initiators. Among such agents may be mentioned the peroxy catalyst suchas the acryl peroxides, e. .g., 'benzoyl peroxide, acetyl peroxide, lauroyl peroxide, etc. and the alkyl peroxides such as diethyl peroxide or tbutyl hydroperoxide. Other suitable agents are photo-activators like benzoin or diacetylin conjunction with ultra-violet light. A particularly useful class of free radical-producing cat+ alysts are the azo compounds disclosed in Hunt U. S. Patent 2,471,959, i. e., the compounds having an acyclic azo group, -N::N, bonded to different carbons which are non-aromatic in character and of which at least one is teritary, and in which the tertiary carbon has attached to it through carbon a radical inwhich thethree remaining valences of thelatter carbon are sati'sfied byat least one element of..atomic number 7 or 8 (oxygen lan'd/or nitrogen)- Byffanthe most useful class of :catalysts, because of their outstanding activity at 'low temperatures, are the symmetrical 'azonitriles disclosed in U. S. Patent 2,471,959 already referred to and in application Serial Number 2,552, filed by J. A. Robertson on January 15, 1948. This class of azonitrile catalystslcomprises the organic compounds, having :anacyclic azo group, N' :N bonded to two different carbons which are nonaromatic, i. e., aliphatic or-cyc1oaliphatic, each of said carbons being tertiary, i. e. attached to three other carbonsbysingle valences, and having a cyano group, CN, attached thereto. These compounds thus have the eneraLformu-la R R3 zi ix mason oNRt wherein the R's aremonovalent organic radicals, or two of the His attached to the same carbon form. together a'five-or 'six mem'bered alicyclic ring. Preferably,the 'Rs are hydrocarbon radicalsor hydrocarbon radicalssubmitted with-alkoxy .groups of one to'four carbons. The most effective of these symmetrical azonitrile catalysts are those in which the radicals attached to the azo :nitrogens have. from 4 to 11 In addition to those shown in theexam'ples, suitable catalysts for use in this carbon atoms.

1 (1896); .Hartrnan, Rec. trav. chim. 4'6, 150 r (1827); Chem. Weekblad, 23, 77 (1926); and Box, J. Amer. ChemgSoc. 47,1471 (1925); or by the method described in U. S. "Patent 2,469,858.

The amount of catalyst, .based on the weight of the vinyl formate, may .be as low as 0.001%

underthe reaction conditions, and essentially chemically inert toward :the vinyl formate and the catalyst. The polymerizationsystemshould be substantially anhydrous since vinyl 'formate hydrolyzes rapidly .in the presence of water, i. e., it should not. contain more than atinost 1% "of water, based .on the weight of the vinyl formate.

, There may be used alcohols such as "ethanol,

isopropyl alcohol tor butanol; ether-alcohols such as methoxyethanohketones such as. acetone; esters such as methyl acetate, ethyl acetate or butyl acetate; heterocyclics' such as tetrahydro furanyetc. The preferred .diluentsaare the alkanols of "1- to 4 carbon atoms and particularly the, secondary alkanols of l to 4 carbon atoms, suchy'as isopropyl alcohol and secondary butyl alcohol, since thesexdiluents lead to polyvinyl formate within the optimum range of molecular weights. Mixed solvents may also bezus'ed.

The diluent may be used in amount'as ilow'as %,basedon the weight of the inonom'er, and as high as 300% or even more. Withdiluents which jact as molecular :weight regulators, such as the preferredlow alkanols, it is only necessary to use small amounts, e.;g.,between 5 and by weight :of 'thefrnonoiier, although more can beuse'd if desired. 7

.The polymerization temperature is desirably kept relatively low, a g. below 100 :C., and preferably below 80" C. With active catalysts like the azonitri-les, it is possible to polymerize vinyl formate at temperatures as low as 0 C. "oreven less. ih'e preferred range of polymerlzation'temperature with these catalysts is between and C. With peroxide-type catalysts, the preferred range of polymerization temperature is between 50 and 80 C. I

The polymerization should be interrupted before gelation (solidification, or setting of the polymer mixture) occurs, i. e., before the polymerizationmixture ceases to be fluid and stirrable. This or as high as 3% or even more.- it generally preferred range is between 0.05% and'l'y Polymerization may be carried. out without added diluent, in which case the unchanged monomer acts as diluent and partial solvent .for the polymer. However, the course of theipolymerization is rather dimcult to control under such conditions. It is much preferable in practice to carry out the polymerization in the presence of an added organic liquid miscible with the monomer. It is not necessary'that the'added diluent "be-an active, solvent for'the polymer and, in fact, this is ordinarily not the case since polyvinyl formate has but limited solubility in most organic solvents. In most cases, thereioregat least partof the polymer separates from the liquid phase as a viscous mass highly swollen with monomer and diluent. The choice of a diluentisnotvery critical since it is only necessary that it be a liquid miscible with the monomer; iin-polymerizable is to prevent undesirable chain branching and production of polymers from which water-resistant fibers cannot be obtained. 'It will be understood that it is not possible to specify "accurately forall polymers the exact point at which polymerization should bearrested. As a'generalrule, however, it has been observed thatthisshould be before the'concentration of the polymer in the total'polymerizing mixture exceeds about 60%. It is often desirable to keep the polymer concentration below about 45% inorder to achieve the best results. While the polymerizationcan be interruptedatany desired point before the upper limit of polymer concentration is reached, it is of ceurseuneconomical to do so too early; Thus, ingeneral, the:polymer concentration will be at least 1% and preferably at least 10%. It may me-noted thatwhile polymer yield, 1. e.,the percentage of monomer converted to polymer, could beused as a test of the desired extent of polymerization, it is not'entirely satisfactory to use it as the sole criterion, for the reason thatthe possibility o'fgelatio'n orsetting. of the polymer mixture is afiected :also by. the molecular weight ofthe polymer and the concentrationof theniixture. Thusalthough it is in'general found that the. polymer yieldxdeesxnot exceed about in some cases it iisjpos'si'ble to have conversions iof or even higherv without reaching the. point of-gel ation. The-poiymerization'may be stopped byv any conventional procedure such as by adding one of the'known polymerization inhibitors such as m dinitrobenzene, J'thiourea, etc. Urireaoted vinyl formate can of course be recovered-and used agains The hydrolysis of the polyvinyl formate to polyvinyl alcohol is preferablybarried out either by alcoholysis, i. e.-, ester interchange, or by treatment with water containing an acidic catalyst. The alcoholysis may be carried out with any desired alcohol but it is preferable to use an alkanol of l 'to' 4 carbon atoms, e.;g.,* methanol, ethanol, propanol, isopropanol, n-butanol or the isomeric butanols, or an ether-alcohol such as methoxyethanol, which givesvery good results. The preferred alcohol is methanol. The catalyst is preferably an alkali-metalalkoxide, which for con venience is preferably the alkoxide of the alcohol used in the alcoholysis process, and it is suitablyused in amounts between 0.05 and 5% based on the weight of the polyvinyl ester.

' The alcoholysis proceeds at any temperature but, for apractical reaction rate, a temperature above about 20 C. is desirable, the preferred range being between 50 and 120 C. The conversion is quantitative. The product separates from its alcohol solution as; the hydrolysis proceeds and it can be washed with suitable nonsolvents such as alcohols to remove the catalyst which may be present. The products containin general not more than at most 1% of nonhydrolyzed formate groups, and usually less. This is evidenced by the fact that their saponifloation number does not exceed Under the conditions described, thepolyvinyl' alcohols will have relatively low molecular weights, as judged from'their viscosity in solution. 'Thepolyvinyl alcohols giving water-resistant fibers have a rela- I tive viscosity in85% phenol at 25 C. and 0.1% concentration not exceeding 1.3. Preferably the relative viscosity is in the range between 1.1 and 1.3. I r

I-Iydrolysis in aqueous media is preferably carriedputwith sulfuric acid as the catalyst, although" other strong acids of dissociation constant above 1X 10 e. g. phosphoric acid, may be used; The hydrolysis temperature is .prefer ably between '75 and 100 C., and it is desirable m use a wetting agent, preferably a sulfonate or sulfate-type wetting agent. I This method has the great technical advantages that, after completion of-thehydrolysis and steam-stripping of the formic acid liberated followed by neutralization with an alkali metal hydroxide, the polyvinyl alcohol jso'lutionmay be spun directly without isolating the polymer.

The polyvinyl "alcohol isspun from aqueous solutions either by the 'dry spinning or the wet spinning process. While dry spinning has certain advantages, such as permitting a high rate of extrusion, it involves certain complications not present in wet spinning, which is therefore preferred. Wet spinning of the polyvinyl alcohol is carried out from aqueous solutions in which the solvent may be water alone but is preferably water" containing a stabilizing agent, i. e., an agent which prevents or decreases the forma tion' of gel particles on standing. There may be used, for instance, between 0.05 and 0.5% of a phosphate ester as' shown in ExamplesII and III, or larger amounts, e. g. about 5% to of a water-soluble amine such aspyridine or watersoluble aliphatic alcohol such as ethanol. Wet-' tingand softening agents may also be included.

Preferably, the solvent should comprise at least 80% water. The concentration of the polyvinyl alcohol in the spinning solution depends to some extent on its molecular weight. It can be as high as desired, provided the solution is fluid enough to handle. Concentrations between5 and are usually employed, an 'optimum range being 10-20%. The spinning solution may be used at any temperature between about 10 C. and its boiling point, i. e., about 100 C., or higher if dry spinning is used.

The coagulating bath is an aqueous solution of one or more inorganic alkali metal salts such as sodium sulfate, disodium hydrogen phosphate, zinc sulfate, potassium sulfate, ammonium chloride, ammonium sulfate, etc. Particularly desirable in view of its better coagulating properties is a bath consisting of monosodium dihydrogen phosphate (NaHzPOi). The salt bath should have a high concentration in. order to remove water rapidly from the aqueous polyvinyl alcohol solution. Preferably the salt concentration is just below the saturation point. Cationic surface-active agents such as cetylpyridinium bromide or cetyltrimethylammonium bromide in amounts of from 0.005 to 0.05% based on the weight of the bath are advantageous in that they promote better spinneret performance and de crease sticking or breaking of filaments. The bath is maintained at any desired temperature between about 5 and about C., preferably between 20 and 80 C.

The coagulated filaments contain salt from the coagulating bath, which salt must be removed before further processing. The filaments may be washed with a water-organic solvent mixture such as a 1:1 mixture of Water and acetone, as described in U. S. Patent 2,388,325. A more economical method is that described in U. '5.

Patent 2,474,617. This method, whichis that used in the examples above, consists in stretching the wet spun yarns while still wet with coagulating bath, holding the filaments, at a fixed length. at least twice their original length and washing them in cold water while so held. Stretching the wet yarn is done either in the bath itself or in air by means of appropriate devices such as a Godet wheel and wind-up wheel or a pair ofdraw rolls. The result of the stretching is to impart to the yarn sufficient orientation to decrease its sensitivity to water to the point where the yarn shows little or no sticking during washing anddrying. In practice, the yarn should be stretched at least 200% of its initial length, i..e., at a draw ratio of 3/1. The draw ratio can be as high as desired up to the point where excessive filament breaking occurs, which is at a draw ratio ofabout-l/l. Washing and drying are carried out at fixed filament length, which is most conveniently achieved while the yarn is on a bobbin. The yarn is washed with water preferably at a temperature not above 25 C. until the, salts are substantially completely removed' as judged by the pH of the 'eiiiuent," ash determination or other methods. Presence of residual salt on-the yarn is detrimental to subsequent operations such as twisting, drawing, etc.

Before the hot drawing step, the yarn should .be relatively dry, that is, in equilibrium with air with a relative humidity of less than 60% at 25C. The yarn may .be dried by a variety of means, e. g. in an oven at about 60 C., or over' a moisture-absorbing material at ordinary temperature'and reduced pressure, or by passing over a hot rod or through a hot zone, e. g. a

tube at C. Since the dried yarn absorbs moisture rapidly, it should be protected from humidity if it is not to be used shortly after drying. Before'hot drawing, the yarn is pref-- erably pretwisted. about 1 to 2 turns per inch. This operation is. by no means essential. but it appears to be beneficial in the subsequentdrawmg. I

The hot drawing is carried out most economically in air but, if desired, in someinert atmosphere such as nitrogen. Any suitable means of uniform heating may be employed, the preferred one being a tube, jacketed with a heattransfer agent or electrically heated, through which the yarn is run. Temperaturesare measured by means of a thermocouple inserted in the tube. The length. of the tube depends on the rate or" drawing. In general,-a contact time of 1 to- 4 seconds gives the best results, and drawing. tensions between 1 and 2 g./d. indicatev a proper combination of speed and temperature. The drawing temperature, 1. e., the temperature of the zone through which. the yarn is passed during stretching, is maintained in a range between 200" C. and 240 C. Thev optimum temperature is readily determined from the experimental limitations that. too high a. temperature causes sticking of the filaments. while too low a temperature causes excessive tension result.- ing in broken filaments. The best results are obtained in general by stretching at a. temperature between 220 and 235 C. and at the. maximum possible tension short of the point where excessive filament breaking occurs. is best carried out by means of a pair of rollers revolving at different peripheral speeds. The draw ratio is desirably at least 2.5/1 and preferably at least 3/1, the maximum permissible .be-

ing in general 4.5/1 for'wet-spun fibers but up to /1 for dry spun fibers.

Polyvinyl. alcohol yarns derived from polyvinyl'iormate according to the method described herein are resistant to boiling water after the thermal stretching step, and do. not require. any additional treatment. However, their water. in.- solubility, resistance to shrinkage and tenacity loss temperature can be further improved by an additional heat-relaxing step. Relaxation of the filament oryarn may be compared to the familiar annealing process. It is believed that it releases strains in the filament and induces a higher degree of crystallinity. The relaxing procedure consists in heating the filamentuncler controlled tension at a temperature which is preferably the same as or somewhat higher than the stretching temperature. The relaxing temperature, however, can be'as' much as 10 C. lower than the stretching temperature. The tension on the filament is such that it pro-- duces no additional drawing and preferably permits shrinking up to a maximum of about 35%, i. e., up to the point where the filament can shrink to 65% of its drawn length. Preferably, the filament or yarn is allowed to shrink between 5% and of its initial length. This is best carried out by running the yarn or filament again through the drawing cell used in the stretching step, or a similar device. capable of being'heated, with the take-up roll at the same or lower peripheral speed than the feed roll. The relative peripheral speeds of the. rolls are adjusted, so that either no shrinking takes place, or preferably, shrinking up to of the initial length. It is only with'yarn from. polymers having the highest degree of linearity that this step can be carried out with no shrinkage and still give a water resistant yarn. The relaxing temperature should be within the range of. 2 10 to 255 C.,' the most generally usefulrange. being 225-245 6. Within these limits, the relaxing Stretching '12 temperature may, in certain. cases, ,b.e as: much as 10 C. below the stretching temperature, but

preferablyit is'the same or a few degreeshigher.

The time during which. thezyarn. willbe in the heated zone canrangefrom, about one second up to the time, usually not more than 30'. seconds, at whichdamage to thefilament, e. g. dis.- coloration and sticking, would result upon con tinued heating. A generally suitable heating time is 2. to 5 seconds- The yarns obtained by the process of. this invention are insoluble in. water at 100 C. Their tenacity usually runs from: 6- to 8 g./d-. with'l5 to 20% elongation. The melting point (for a highly crystalline material this term is substantially synonymous withsoftening point or sticking point) of the finished polymer shows no appreciable change from that of the original polymer, 1. e., itis. 212i5 C. The polyvinyl alcohol filaments. contain at least 99% vinyl alcohol units, any remainder being vinyl formate units.

The polyvinyl alcohol yarns obtained according tothis process may bev used for any textile application suchas knitting, weaving, etc. More specifically, the yarns may be used in shirting, sheetings, awnings, canvas, woven hose, in the reinforcing of rubber goods such as tires and tubes, in womens apparel, industrial fabrics, etc- The foregoing. detailed description has. been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is. not. limited to the exact details shownand described for obvious modifications will occur to those skilled in the art. I

What i'sclaimed is: I

l. A process for the preparation of improved polyvinyl alcohol. fibers. which comprises polymerizing vinyl. formate in. a, liquid diluent miscible with vinyl formate monomer, interrupting the polymerization (a), before the concentration of the polymer in the. polymerization mixture exceed's of they/eight of the totalmixture and (1)) before theconve-rsion of monomer to polymer exceeds hydrolyzing the polymer.v to polyvinylalcohol of saponification number not exceeding l0, spinning an aqueous solutionof the polyvinylalcohol to form a filament,v orienting said filament tov a dry oriented.- length at least three times as longasthat of the. spun. filament, and drying, and further orienting, the d-ry-"filament by stretching the same at. a' temperature between200v and 240 C. to a fina-l length atleasttwo and onehalf times thelength after thefirst orientation. I

2. Aprocess for the preparation or improved polyvinyl alcohol fibers which comprises polymerizing. vinyl formats in contact with a polyinerization catalyst, interrupting the; polymerizae tionlal before; the concentration of the polymer in the polymerization mixture exceeds 60% of the weight of. thetotal mixture and (1))v before the: conversion'of monomer to polymerv exceeds 75 hydrol-yzing; the polymer to. polyvinyl alcohol of sap-onification.numberinot. exceeding 10, spinning an aqueous. solution. of the polyvinyl alcohol to form a filament, orienting said filament to aclry oriented length at least three times as long as that of the: spunfilament, and drying, and further orienting the dry filament by stretching the same at atemperature between 200" and 240 C. to afinallength at least two and one-half times the length after the first orientation.

3. Aprocess for the preparation of improved said filament to a dry oriented length at least three times as long as that of the spun filament, and drying, further orientingthe dry filament by stretching the same at a temperature between 200 and 240 C. to a final length at least two and one-half times the length after the first orientation, and relaxingv said further oriented fiber by heating the same at a temperature between 210 and 255C. but not more than C. below the temperature at which it was further oriented and under tension controlled to prevent shrinkage of more than 35% of said further oriented length and untilthe filament shrinks less than 10% of its length when immersed for six minutes in water at 100 C.

4. A process fort'ne preparation of improved polyvinyl alcohol fibers which comprises polymerizing vinyl formate in a liquid diluent miscible with vinyl formate monomer, interrupting the polymerization (a) before the concentration of the polymer in the polymerization mixture exceeds 60% of the weight of the total mixture and (b) before the conversion of monomer to polymer exceeds 75%, hydrolyzing the polymer to polyvinyl alcohol of saponification number not exceeding 10, spinning an aqueous solution of the polyvinyl alcohol into an aqueous salt solution coagulating bath to form a filament, orienting said filament to a dry oriented length at least three times as long as that of the spun filament, washing said filament to remove coagulating bath salts, drying said filament and further orienting the dry filament by stretching the same at a temperature between 200 and 240 C. to a final length at least two and one-half times the length after the first orientation.

5. A process for the preparation of improved polyvinyl alcohol fibBI'S,'WhiOh comprises polymerizing vinyl formate in contact with a polymerization catalyst, interrupting the polymerization (a) before the concentration of the polymer in the polymerization mixture exceeds 60% of the weight of the total mixture and (5) before the conversion of monomer to polymer exceeds 75%, hydrolyzlng the polymer to polyvinyl alcohol of saponification number not exceeding 10, spinning an aqueous solution of the polyvinyl alcohol into an aqueous salt solution coagulating bath to form a filament, orienting said filament to a dry oriented length at least three times as long asthat of the spun filament, washing said filament to remove coagulating bath salts, drying said filament and further orienting the dry filajment by stretching the same at a temperature between 200 and 240 C. to a final length at least two and one-half times the length after the first orientation.

6. A process for the preparation of improved polyvinyl alcohol fibers which comprises polymerizing vinyl formate in a liquid diluent miscible with vinyl formate monomer, interrupting the polymerization (a) before the concentration of the polymer in the polymerization mixture exceeds of the weight of the total mixture and (h) before the conversion of monomer to polymer exceeds hydrolyzing the polymer to polyvinyl alcohol of saponification number not exceeding 10, spinning an aqueous solution of the polyvinyl alcohol into an aqueous salt solution coagulating bath to form a filament, orienting said filament to a dry oriented length at least three times as long as that. of the spun filament. washing said filament to remove coagulating bath salts,'drying said filament and further orienting the dry filament by stretching the same at a temperature between 200 and 240 C. to a final length at least two and one-half times the length after the first orientation, and relaxing said further oriented fiber by heating the same at a temperature between 210 and 255 C. but not more than 10 C,' below the temperature at which it was further oriented and under tension controlled to prevent shrinkage of more than 35% of said further oriented length and until the filament shrinks less than 10% of its length when immersed for six minutes in water at C.

WILLIAM R. HATCHARD. JULIAN W. HILL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Jones, Polyvinyl Alcohol, British Plastics, February 1944, pp. 77-83, 18-P. V. A. Digest. 

1. A PROCESS FOR THE PREPARATION OF IMPROVED POLYVINYL ALCOHOL FIBERS WHICH COMPRISES POLYMERIZING VINYL FORMATE IN A LIQUID DILUENT MISCIBLE WITH VINYL FORMATE MONOMER, INTERRUPTING THE POLYMERIZATION (A) BEFORE THE CONCENTRATION OF THE POLYMER IN THE POLYMERIZATION MIXTURE EXCEEDS 60% OF THE WEIGHT OF THE TOTAL MIXTURE AND (B) BEFORE THE CONVERSION OF MONOMER TO POLYMER EXCEEDS 75%, HYDROLYZING THE POLYMER TO POLYVINYL ALCOHOL OF SAPONIFICATION NUMBER NOT EXCEEDING 10, SPINNING AN AQUEOUS JSOLUTION OF THE POLYVINYL ALCOHOL JTO FORM A FILAMENT, ORIENTING SAID FILAMENT TO A DRY ORIENTED LENGTH AT LEAST THREE TIMES AS LONG AS THAT OF THE SPUN FILAMENT, AND DRYING, AND FURTHER ORIENTING THE DRY FILAMENT BY STRETCHING THE SAME AT A TEMPERATURE BETWEEN 200 AND 240* C. TO A FINAL LENGTH AT LEAST TWO AND ONE-HALF TIMES THE LENGTH AFTER THE FIRST ORIENTATION. 